Aliphatic polyesters, such as polyglycolic acid and polylactic acid, can be decomposed by microorganisms or enzymes present in nature, such as soil or sea water, so that they are noted as biodegradable polymer materials giving little load to the environment. Further, aliphatic polyesters are utilized as polymer materials for medical use, such as sutures for surgery or artificial skin, since they can be decomposed or absorbed in vivo.
Among the aliphatic polyesters, polyglycolic acid is excellent in gas-barrier properties, such as oxygen gas-barrier property, carbon dioxide gas-barrier property and water vapor-barrier property and also excellent in heat resistance and mechanical properties, and therefore the development of new use thereof is under way singly or in a composite state together with another resin material in the fields of packaging materials, etc.
An aliphatic polyester can be synthesized by dehydro-polycondensation of an α-hydroxycarboxylic acid, such as glycolic acid or lactic acid, but in order to effectively synthesize an aliphatic polyester of a high molecular weight, there has been generally adopted a process of synthesizing a bimolecular cyclic ester of an α-hydroxycarboxylic acid and subjecting the cyclic ester to ring-opening polymerization. For example, by ring-opening polymerization of glycolide that is a bimolecular cyclic ester or cyclic diester of glycolic acid, polyglycolic acid is obtained. By ring-opening polymerization of lactide that is a bimolecular cyclic ester of lactic acid, polylactic acid is obtained.
A cyclic ester generally contains impurities including free carboxylic acid compounds, such as an α-hydroxycarboxylic acid used as the starting material and linear α-hydroxycarboxylic acid oligomers, and water. As impurities, such as water, even in a minute amount, can adversely affect the ring-opening polymerization of a cyclic ester, it has been proposed to use a high-purity cyclic ester from which impurities have been removed as far as possible in the ring-opening polymerization.
On the other hand, an alcohol such as a higher alcohol has been used as a molecular weight-adjusting agent in ring-opening polymerization of a cyclic ester in order to control the molecular weight of the aliphatic polyester. There has been also proposed a method of determining an addition amount of the alcohol based on the amount of free carboxylic acid compounds contained in the cyclic ester.
For example, in the ring-opening polymerization of glycolide, there has been proposed heretofore a method of using substantially pure glycolide purified by re-crystallization, etc. and also using a higher alcohol such as lauryl alcohol as a molecular weight-adjusting agent (e.g., Patent document 1b listed below).
Further, a purification method of removing impurities such as water from a cyclic ester (e.g., Patent document 2 listed below). In this document, it is pointed out that impurities, such as water, α-hydroxycarboxylic acid and low-molecular weight oligomers thereof, contained in a cyclic ester, should be removed, since they exert various functions as an initiator, a chain transfer agent, a catalyst deactivator, etc., to obstruct the ring-opening polymerization.
There has been proposed a process for producing an aliphatic polyester by ring-opening polymerization of a cyclic ester having a water content of at most 80 ppm and an acid value of at most 0.10 mg KOH/g (e.g., Patent document 3 listed below). This document contains a description to the effect that the reduction of water content in a cyclic ester accelerates the polymerization speed and allows the production of a high-molecular weight polymer, and the presence of an alcohol in the polymerization system suppresses the function of water content to allow the production of an aliphatic polyester of a good quality.
As a process for producing an aliphatic polyester by ring-opening polymerization of a cyclic ester, there has been proposed a production process characterized by determining the amount of a hydroxyl compound added to the reaction system based on the amount of free carboxylic acid compounds contained in the cyclic ester (e.g., Patent document 4 listed below). The document discloses α-hydroxycarboxylic acid used for production of the cyclic ester and linear oligomers of the α-hydroxycarboxylic acid as the free carboxylic acid compounds, and describes that monohydric linear saturated alcohols having 12-18 carbon atoms are preferred as the hydroxyl compound.
The document points out that if impurities, such as water and free carboxylic acid compounds, are contained in a cyclic ester, they adversely affects the polymerization reaction and it becomes impossible to effect a targeting, i.e., production of a polymer with an objective molecular weight, even under identical polymerization conditions. The document describes that the control of a molecular weight of aliphatic polyester is liable to be difficult at a large water content and it is preferred to control the water or moisture content in the cyclic ester to at most 100 ppm in order to accurately control the molecular weight.
The document further describes that water in a cyclic ester can be easily removed by purification and drying steps immediately before the polymerization, but free carboxylic acid compounds are difficult to remove and greatly affect the polymerization, and moreover, a cyclic ester is liable to cause ring-opening due to a minute amount of water during the storage to produce free carboxylic acid compounds anew. The document has proposed a process for producing an aliphatic polyester of a target molecular weight by determining the amount of free carboxylic acid compounds contained in a cyclic ester and adding an amount of hydroxyl compound (e.g., a higher alcohol) corresponding thereto.
Patent document 1: U.S. Pat. No. 3,442,871
Patent document 2: JP-A 8-301864
Patent document 3: JP-A 10-158371
Patent document 4: JP-B 3075665
Patent document 5: JP-A 2001-261797
As described above, water has been recognized as an impurity obstructing the ring-opening polymerization of a cyclic ester and should be removed as far as possible. However, water is the most universal compound present in nature, and the removal thereof as an impurity is confronted with a certain limit. The present inventors, et al, performed a detailed study about the function of water in the system of ring-opening polymerization of a cyclic ester and, as a result thereof, found it possible to control the molecular weight of a resultant aliphatic polyester by using a proton-source compound including water as a molecular weight-adjusting agent and controlling a total proton concentration in the cyclic ester to smoothly proceed with the ring-opening polymerization of a cyclic ester. Based on the knowledge, a process for producing an aliphatic polyester has been already proposed (WO2004/033527A).